The long-term aim of my research is to understand the neural underpinning of visual perception in man. Although it is well established that the primate visual system employs two distinct visual streams, the parvocellular-projecting (P) and the magnocellular-projecting (M) streams, their respective roles in vision are still controversial, largely because recent research has raised fundamental questions regarding the organization of the receptive fields of neurons in these streams. The thorniest of these questions include the linearity of center/surround interactions in P cells' receptive fields, and the nature of the photoreceptors' input (both cones and rods) to the center and surround of P and M receptive fields. In addition, our understanding of the retinal mechanisms which account for the different properties of the cells in the M and P streams is partial at best. The studies proposed here will address these issues by investigating, in quantitative detail, the functional organization of receptive fields of visual neurons in the primate retina and Lateral Geniculate Nucleus (LGN), using macaque monkeys whose visual system is very similar to that of humans. Retinal ganglion cell activity will be monitored extracellularly as synaptic (S) potentials recorded together with LGN responses in the LGNs of anesthetized, paralyzed monkeys. Visual stimuli designed to excite one or more classes of cones in the center or surround regions of the receptive field will be temporally modulated by an 'm-sequence' of pulses or by a sum-of-sinusoids, to extract information about both linear and nonlinear aspects of the response dynamics. The proposed study will a) investigate the temporal, spatial and chromatic properties of the recently-discovered nonlinear (divisive) surround region of the receptive fields of P cells, b) determine if the surround mechanism of P cells has both linear and nonlinear components, c) determine whether the cone input to the surrounds of P receptive fields is characteristically pure or mixed, d) determine the degree to which rods contribute to the response of P and M cells, and e) compare the processing of color information in retinal ganglion cells with that of their LGN targets. These studies will resolve current controversies, and provide new insights into the roles that the M and P streams play in vision and into the cellular mechanisms which determine their responses and capacities. Such insights will be important for the understanding of both normal and pathological aspects of human vision.